In other worships, Allah (SWT) commanded His servants to do it. But especially in the command to read shalawat, Allah almighty mentions that Allah himself prayed to the Prophet (ﷺ ), as well as His angels, only then commanded those who believed to pray for him. With this increasingly showing that his worship of the Prophet Muhammad (ﷺ ), is the main worship.

If we want to know that prayer is the main worship, then observe and reflect on the word of Allah SWT in the Qur'an: "Verily Allah and His angels, pray for the Prophet, O believers, bless you for the Prophet and say praise to him". (QS. Al-Ahzab 56).

From this verse we know, Allah SWT alone the Creator and also the angels prayed to the Prophet (ﷺ ), why we as our ummah he does not want to pray to him. 

Penetrating Nature

Shalawat is a worship that is not limited to nature, distance or time. This means that when it is spoken it will penetrate the very distant sky, be heard by the angels, and then also convey a prayer for the man who said it, and through the grave conveys the greeting spoken by man to the Prophet Muhammad (ﷺ ).

The Prophet said: "None of you greeted me after I died but the angel Gabriel came to me saying: 'O Muhammad, this is Fulan bin Fulan saying greetings to you, so I replied: "and upon it greetings and mercy and blessings from Allah". (HR. Ashes of David)

Similarly, the companions of Abu al Darda RA said, "Pray for me on Friday, for he is witnessed by angels. No one worships me unless he is presented to me until he has finished praying. 

"I asked, "Even after you die?" The Prophet PBUH replied, "Even after I have died. Indeed Allah has forbidden the earth from the body of the Prophets, but the Prophet (saw Allah) is alive therein." 

It is narrated by Ahmad and Abu Daud of Abu Hurairah RA, where the Prophet PBUH said: "No one greets me unless Allah returns my ruhku to me until I can answer his greetings."  

As for al Nasa'i and others, the Prophet PBUH said: "Indeed, Allah has delegated several angels in my grave to convey my greetings from my ummat."